In recent years, a trend that thawing of foods by the use of high-frequency heating apparatuses is automated has become predominant. Conventionally, a procedure has been followed in which the weight of a food is inputted by keys to control automatic timing or the weight is measured by using a weight sensor for automatically detecting the weight of the food such that the food is heated for a corresponding one of optimum heating periods preset for different weights of foods, respectively.
Furthermore, Japanese Patent Publication No. 52-2133 proposes that a microwave detecting element, namely, an antenna be disposed in a heating chamber so as to utilize a characteristic that the electric power of microwaves detected by the microwave detecting element which have not been absorbed by a food is inversely proportional to the weight of the food. Hereinbelow, this arrangement is described with reference to FIG. 1. A frozen food 2 is placed in a heating chamber 1 and waves 4 are irradiated over the food 2 from a wave irradiator 3. At this time, a portion 5 of the waves, which has not been absorbed by the food 2, is sensed by an antenna 6 mounted in the heating chamber 1 and then is subjected to detection by a detection circuit 7 so as to be delivered to a controller 8. Since the quantity of the waves sensed by the antenna 6 is inversely proportional to the amount absorbed by the food 2, the weight of the food 2 can be judged and thus, its optimum heating period can be set.
However, in the known high-frequency heating apparatus referred to above, in the case where state of waves in the heating chamber is sensed by using the antenna and the detection circuit in order to detect state of the food in the heating chamber, the detection level changes greatly depending on the construction of the antenna, construction of the detection circuit, mounting accuracy of the antenna and the detection circuit, influence of any external magnetic field, etc. Thus, a problem arises that this constitutes an extremely unstable factor for determination of subsequent sequences requiring estimation of the weight of the food, etc.